fix(stac-api): next link and trailing slash bug in stac api url

[anayeaye]

What

Fix problem in which a stac-fastapi deployed behind a proxy (i.e. customdomain.com) with a custom root path (i.e. /api/stac)

• Fixed: API does not properly resolve customdomain.com/api/stac -> customdomain.com/api/stac/
• Fixed: API does not properly use custom root path in next links of paginated responses customdomain.com/api/stac/... links are returned as customdomain.com/... without the custom root path.

The correction is to remove the gateway configuration from the Mangum ASGI handler for the stac-api handler.

Refs

• Fastapi behind a proxy
• Mangum configuring an adaptar instance with api_gateway_base_path pattern that we use and need for titiler/raster-api and ingest-api

Issue

#343

Testing?

• I deployed this change manually to the dev veda-backend stack and confirmed that the stac-api url is properly resolved and that the next links now properly include the root path.